Generally, a motor vehicle powershift transmission includes a number of gear elements coupling the input and output shafts, and a related number of clutches that are selectively engageable to activate gear elements for establishing a desired speed ratio between the input and output shafts. The clutch may be of the band or disk type.
For example, the input shaft may be connected to the engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle drive. Shifting from one gear ratio to another involves releasing or disengaging the off-going clutches associated with the current gear ratio and applying or engaging the oncoming clutches associated with the desired gear ratio by controllably actuating and deactuating a plurality of solenoid actuators.
Transmissions of this type are often electronically controlled and include diagnostic functions for determining when a fault condition exists. For example, diagnostic systems can sense electrical problems in the control solenoids. A system of this type is illustrated in U.S. Pat. No. 4,414,863 issued Nov. 15, 1983 to Heino. Diagnostic systems have also been developed to sense mechanical problems, such as clutch non-engagement, in response to sensing clutch pressure. A system of this type is illustrated in U.S. Pat. No. 5,174,137 issued Dec. 29, 1992 to Kato et al. While systems of this type are operable, pressure sensors or switches are often unreliable and expensive. Similarly, systems of this type include not only pressure sensors, but also input, intermediate, and output speed sensors.
The present invention is directed to overcoming one or more of the problems as set forth above.